This invention relates to an automatic compression relief mechanism for internal combustion engines, and more particularly to a centrifugally responsive compression release mechanism.
It is often desirable to reduce the amount of compression in a combustion chamber at engine cranking speeds to facilitate starting of the engine. In pull-starting engines, for example, reduced compression in the combustion chamber lessens the amount of operator effort in pulling the engine pull rope. The engine is easier to start, and operator fatigue is minimized.
In a conventional compression release mechanism, an auxiliary cam member engages a cam follower at engine cranking speeds to partially unseat either the intake valve or the exhaust valve to relieve compression in the combustion chamber. At higher engine speeds, the auxiliary cam member moves to an inoperative position so that it does not engage the cam follower and the valve is not unseated by the auxiliary cam member. At these higher engine running speeds, the valves are cyclically unseated by a cam affixed to a camshaft, which rotates in timed relation to the engine crankshaft.
It is apparent that the cam follower and the valve operating means impart a force or a load upon the auxiliary cam member. In some prior art compression release mechanisms, this load is borne by the pivot pins which pivotally connect the compression release member to the camshaft. Such devices have the disadvantage that the imparted loads tend to result in an excessive wear on the pivot pins as well as on the pivot pin retainer holes in the compression release mechanism. This excessive wear causes the compression release mechanism to become less effective or to fail all together. The pivot pins may even break off due to the imparted loads.
Braun et al U.S. Pat. No. 4,453,507 issued Jun. 12, 1984 and assigned to Briggs and Stratton Corporation, the assignee of the present invention, discloses a compression release mechanism in which the load imposed by the valve operating means is spaced from the pivot pin. As disclosed in Column 3, Lines 52 transferred to the camshaft via a load bearing surface located at the base of the auxiliary cam member.